The application of specific electrical energy to the spinal cord for the purpose of managing pain has been actively practiced since the 1960s. It is known that application of an electrical field to spinal nervous tissue can effectively mask certain types of pain transmitted from regions of the body associated with the stimulated nervous tissue. Such masking is known as paresthesia, a subjective sensation of numbness or tingling in the afflicted bodily regions. Application of electrical energy has been based on the gate control theory of pain. Published in 1965 by Melzack and Wall, this theory states that reception of large nerve fiber information, such as touch, sense of cold, or vibration, would turn off or close the gate to reception of painful small nerve fiber information. The expected end result would, therefore, be pain relief. Based on the gate control theory, electrical stimulation of large fibers of the spinal cord cause small fiber information to be reduced or eliminated at that spinal segment and all other information downstream from that segment would be reduced or eliminated as well. Such electrical stimulation of the spinal cord, once known as dorsal column stimulation, is now referred to as spinal cord stimulation or SCS.
FIGS. 1A-1B illustrate conventional placement of an SCS system 10. Conventional SCS systems include an implantable power source or implantable pulse generator (IPG) 12 and an implantable lead 14. Such IPGs 12 are similar in size and weight to pacemakers and are typically implanted in the buttocks of a patient P. Using fluoroscopy, the lead 14 is implanted into the epidural space E of the spinal column and positioned against the dura layer D of the spinal cord S, as illustrated in FIG. 1B. The lead 14 is implanted either through the skin via an epidural needle (for percutaneous leads) or directly and surgically through a mini laminotomy operation (for paddle leads).
FIG. 2 illustrates example conventional paddle leads 16 and percutaneous leads 18. Paddle leads 16 typically have the form of a slab of silicon rubber having one or more electrodes 20 on its surface. Example dimensions of a paddle lead 16 is illustrated in FIG. 3. Percutaneous leads 18 typically have the form of a tube or rod having one or more electrodes 20 extending therearound. Example dimensions of a percutaneous lead 18 is illustrated in FIG. 4.
Implantation of a percutaneous lead 18 typically involves an incision over the low back area (for control of back and leg pain) or over the upper back and neck area (for pain in the arms). An epidural needle is placed through the incision into the epidural space and the lead is advanced and steered over the spinal cord until it reaches the area of the spinal cord that, when electrically stimulated, produces a comfortable tingling sensation (paresthesia) that covers the patient's painful area. To locate this area, the lead is moved and turned on and off while the patient provides feedback about stimulation coverage. Because the patient participates in this operation and directs the operator to the correct area of the spinal cord, the procedure is performed with local anesthesia.
Implantation of paddle leads 16 typically involves performing a mini laminotomy to implant the lead. An incision is made either slightly below or above the spinal cord segment to be stimulated. The epidural space is entered directly through the hole in the bone and a paddle lead 16 is placed over the area to stimulate the spinal cord. The target area for stimulation usually has been located before this procedure during a spinal cord stimulation trial with percutaneous leads 18.
Although such SCS systems have effectively relieved pain in some patients, these systems have a number of drawbacks. To begin, as illustrated in FIG. 5, the lead 14 is positioned upon the spinal cord dura layer D so that the electrodes 20 stimulate a wide portion of the spinal cord and associated spinal nervous tissue. The spinal cord is a continuous body and three spinal levels of the spinal cord are illustrated. For purposes of illustration, spinal levels are sub-sections of the spinal cord S depicting that portion where the dorsal root DR and ventral root VR join the spinal cord S. The peripheral nerve N divides into the dorsal root DR and the dorsal root ganglion DRG and the ventral nerve root VR each of which feed into the spinal cord S. An ascending pathway 17 is illustrated between level 2 and level 1 and a descending pathway 19 is illustrated from level 2 to level 3. Spinal levels can correspond to the veterbral levels of the spine commonly used to describe the vertebral bodies of the spine. For simplicity, each level illustrates the nerves of only one side and a normal anatomical configuration would have similar nerves illustrated in the side of the spinal cord directly adjacent the lead.
Motor spinal nervous tissue, or nervous tissue from ventral nerve roots, transmits muscle/motor control signals. Sensory spinal nervous tissue, or nervous tissue from dorsal nerve roots, transmit pain signals. Corresponding dorsal and ventral nerve roots depart the spinal cord “separately”; however, immediately thereafter, the nervous tissue of the dorsal and ventral nerve roots are mixed, or intertwined. Accordingly, electrical stimulation by the lead 14 often causes undesirable stimulation of the motor nerves in addition to the sensory spinal nervous tissue.
Because the electrodes span several levels the generated stimulation energy 15 stimulates or is applied to more than one type of nerve tissue on more than one level. Moreover, these and other conventional, non-specific stimulation systems also apply stimulation energy to the spinal cord and to other neural tissue beyond the intended stimulation targets. As used herein, non-specific stimulation refers to the fact that the stimulation energy is provided to all spinal levels including the nerves and the spinal cord generally and indiscriminately. Even if the epidural electrode is reduced in size to simply stimulate only one level, that electrode will apply stimulation energy indiscriminately to everything (i.e. all nerve fibers and other tissues) within the range of the applied energy. Moreover, larger epidural electrode arrays may alter cerebral spinal fluid flow thus further altering local neural excitability states.
Another challenge confronting conventional neurostimulation systems is that since epidural electrodes must apply energy across a wide variety of tissues and fluids (i.e. CSF fluid amount varies along the spine as does pia mater thickness) the amount of stimulation energy needed to provide the desired amount of neurostimulation is difficult to precisely control. As such, increasing amounts of energy may be required to ensure sufficient stimulation energy reaches the desired stimulation area. However, as applied stimulation energy increases so too increases the likelihood of deleterious damage or stimulation of surrounding tissue, structures or neural pathways.
Improved stimulation systems and methods are desired that enable more precise and effective delivery of stimulation energy. At least some of these objectives will be met by the present invention.